BSEP116B Biodiversity in the Baltic Sea - Helcom

Number of species
16
14
12
10
8
6
4
2
0
Tolypella
Nitellopsis
Nitella
Lamprothamnium
Chara
of charophytes is found in the easternmost and
northernmost areas of the Baltic Sea (Figure
3.2.4.). The most common species in the Baltic
Sea is Chara aspera Willd. Chara baltica Bruzelius
Kattegat W
Danish Belts
Kattegat E
Øresund
Zealand and Scania
Gulf of Kiel
Arkona Sea S
Bornholm Sea
Hanö Bay
Polish coast
Gulf of Gdansk
Baltic proper W
Gotland Sea
Baltic proper E
Gulf of Riga
Estonian Arcipelago
Baltic proper NW
Gulf of Finland
Finnish Archipelago Sea
Åland Sea
Bothnian Sea
Bothnian Bay
Figure 3.2.4. Number of Charophyte species in different areas of the
Baltic Sea. Data from Schubert & Blindow 2003, Torn 2008. Division of
the Baltic Sea after Nielsen et al. 1995.
is also distributed all over the Baltic Sea area and
can be found on both sheltered and moderately
exposed coastlines. One of the rare species, Chara
connivens Salzm. Ex A. Braun, is considered to
have been brought to the Baltic Sea by ballast sand
and boulders in the sailing ship era; it has a limited
distribution in the Baltic Sea, but is abundant in the
areas in which it is found (Figure 3.2.5).
In the Baltic Sea, charophytes inhabit mostly sheltered
coastal areas, where their distribution pattern
is primarily controlled by the salinity regime, settlement
depth, sediment type and exposure (Schubert
& Blindow 2003).
In recent decades, the number of species, distribution
area and biomass of charophytes have
declined significantly in the Baltic Sea (Schubert &
Blindow 2003). Most of the records on the considerable
decline of charophyte populations are from
coastal waters of Schleswig-Holstein, the Swedish
west coast and the coastal waters of Hanko peninsula
in southwestern Finland (Shubert & Blindow
2003). The decline of charophytes is mainly caused
by mechanical stress, combined with the destruction
of habitats and human-induced pressures such
as eutrophication.
38
Figure 3.2.5. Present distribution of invasive Chara connivens in the Baltic
Sea (Torn 2008).
Mytilus trossulus (Blue mussel)
Two species of blue mussels can be found in the
Baltic Sea area. Mytilus edulis is distributed in the
North Atlantic and penetrates into most of the
Kattegat. In the Baltic Sea, Mytilus trossulus dominates
the shallow, hard substrates in the deeper
vegetation belt and below, with abundances up to
36 000–156 000 individuals m −2 (Kautsky 1982). In
the northern Baltic Proper, blue mussels are most
often found in the depth range 0–25 m (Westerbom
2006); they have their maximum between
10–15 m depth (Littorin 1998) and usually start to
decline after 30 m depth. Blue mussels dominate
the animal biomass on hard substrates and, owing
to their dominance, are considered to be one of
key functioning species in the Baltic Proper. Quantitatively,
the blue mussel constitutes up to 80–90%
of total animal biomass in the shallow areas of the
Baltic Sea (Kautsky et al. 1990). The blue mussel
is considered to be an important link between
the benthic and pelagic components of the Baltic
ecosystem, channelling the flow of energy and
matter, and being able to filter annually an amount